Drilling holes in concrete, stone, etc, is done by attaching a dedicated drill bit to a rotating hammer drill and by applying both an axial vibrational striking force and a rotational torque to the drill bit. In order to meet demands for high efficiency in a drilling operation, a drill bit is typically constructed such that a cutting blade chip made of cemented carbide which has high wear resistance is fixedly attached to a tip end of the cutting edge of a bit body of the drill bit by brazing, welding, etc.
The applicant has already proposed a drill bit of this type which is used exclusively for drilling holes in concrete, stone, etc, and is constructed such that a cutting blade chip fixedly attached to a lower end portion of a bit body has a plurality of cutting blades, each cutting edge is formed by an edge between a face and a flank of the cutting blades, tip ends of the cutting edges of the cutting blades are coupled at a center thereof to form a pointed shape without a chisel edge, a concave portion formed between the cutting blades is continuous with a chip discharge groove formed on an outer periphery of the bit body (for example, see Japanese Laid-Open Patent Application Publication No. 2002-178328).
Assume that the above mentioned drill bit without the chisel edge is attached to a rotating hammer drill and carries out drilling holes in a concrete, stone, etc. In a cutting operation (striking and cutting operation) carried out by applying an axial vibrational striking force to a crush face of a drill hole, i.e., by striking a tip end of the cutting edge to the crush face of the drill hole, the crush face of the drill hole may be crushed, and then a bit rotates to cut the crush face of the drill bit which has become fragile due to prior striking and cutting. In this manner, the drilling operation progresses.
In this case, the degree to which the crush face of the drill hole is crushed by striking becomes larger at a center portion of the tip end of the cutting edge of the pointed shape into which tip ends of the cutting edges are adapted to gather.
However, when the cutting operation is carried out by rotation of the bit, a cutting ability is low at the center portion of the tip end of the cutting edge because rotation track is not substantially drawn. In addition, since chips are inefficiently discharged from a center portion of the crush face of the drill hole which is conically recessed, they tend to remain there. The center portion of the cutting edges repeatedly strikes the remaining chips. This reduces cutting efficiency of striking and cutting.
Accordingly, an object of the present invention is to provide a drill bit capable of improving a tip end portion of cutting edges to reduce chips remaining in a center portion of a crush face of a drill hole to thereby improve cutting efficiency.
In order to solve the above described problem, there is provided a drill bit comprising a cutting blade chip formed by a cemented carbide block body, the cutting blade chip having a plurality of cutting blades arranged in a circumferential direction of the cutting blade chip to extend radially outward, the cutting blades being each structured to have a cutting edge formed by an edge between a face and a flank of the cutting blades, the cutting edges extending radially outward such that tip ends are positioned on a tip end portion of the cutting blade chip, wherein a groove is configured to be provided in a center portion of the cutting blade chip in contact with tip ends of the cutting edges, and the tip ends of the cutting edges have pointed portions.
In accordance with the drill bit constructed as described above, the tip ends of the cutting edges are formed in contact with the groove and have a pointed shape. Therefore, when the drill bit is attached to the rotating hammer drill and used for drilling holes in concrete, stone, etc, it strikes the crush face of the drill hole. In this case, the pointed portions at the tip ends of the cutting edges which are close to each other strike the crush face of the drill hole at the same time. For this reason, the crush face of the drill hole is also crushed to a region corresponding to the pointed portions (groove portions), and chips being generated at the crush face of the drill hole are left in the groove. In this state, when the drill bit continues to rotate, the pointed portions of the tip ends of the cutting edges draw a rotation track. In the rotation and cutting operation performed by the cutting edges, the entire cutting edges (over an overall length) entirely cut the crush face of the drill hole which has become fragile due to prior striking and cutting. In addition, the chips left in the groove after prior striking and cutting operation is discharged outside the groove without substantial resistance by the rotation of the groove. Since the chips remaining in the center portion of the crush face of the drill hole is reduced, cutting efficiency improves.
The groove may be formed to cross an axis (axis extending in a longitudinal direction of the drill bit) of the drill bit at a substantially right angle, the groove may be structured to have a face of a circular-arc shape as viewed from a side such that a width of the groove increases from a deepest point thereof toward the tip end of the cutting edge. In this structure, a circular-arc face of the groove is inclined toward the tip end of the cutting edge, and the pointed portion of the tip end of the cutting edge is formed substantially by an apex of three inclined faces including the face and the flank of the cutting blade, and the circular-arc face of the groove. This increases resistance to impact. Therefore, deformation or wear of the pointed portions of the tip ends of the cutting edges are reduced, regardless of repeated striking and cutting operation with respect to the crush face of the drill hole. As a result, cutting performance is stable. Further, since the groove has the circular-arc face, the chips easily move in the groove and are easily discharged from the groove.
While the number of the cutting blades is not specifically limited in the drill bit of the present invention, the cutting blades may be formed by three cutting blades arranged at uniform (equal) intervals in a circumferential direction thereof, the three cutting blades may be structured such that the tip end of the cutting edge of one of the three cutting blades is positioned at one end portion of the groove, and the tip ends of the cutting edges of remaining cutting blades are positioned at intermediate portions of the groove, and an opposite end portion of the groove is positioned in a concave (trough) portion formed between a face and a flank of associated cutting blades. In this structure, the concave portion in which the end portion of the groove is positioned leads to the chip discharge groove formed on the outer periphery of the bit body through the concave portion formed between the cutting blades. Therefore, the chips are efficiently discharged from the groove.
The drill bit provided with the groove on the tip end side of the cutting edge is manufactured in a simple process in such a manner that the tip ends of the cutting edges are coupled to each other at the tip portion of the cutting blade chip to form the pointed portions and then the groove is formed on the tip end side of the cutting edge. As a result, a drill bit (product) which has cutting edges and pointed portions with a high finish precision and has high quality is easily manufactured with high productivity.